From at least the 6th millennium BC on a trend can be observed towards increasing regional divergence in the cultural assemblages of the Mesopotamian alluvium and the neighboring plains of Khuzestan in southwestern Iran. This millennia old trend was sharply reversed sometime in the Uruk Period of the millennium when communities in each of the two areas began to develop in increasingly analogous ways. This is clearest in the case of the Susiana Plain, the largest, most productive, and historically most important.

By the later part of the Uruk sequence the Susiana plain had become part and parcel of the Mesopotamian world, an eastward extension of the culture and institutions prevalent in southern Iraq. This cultural convergence is too pervasive to be explainable as a process of acculturation. Rather it is indicative of the colonization of the Khuzestan plains by settlers from the alluvium and represents an early phase of the expansion of Mesopotamian societies in the Uruk Period. The end result of this process was the creation of at least two rival states centered respectively at Susa and Chogha Mish. These appear to have been independent of each other since by the end of the Uruk Period each site was surrounded by numerous subsidiary centers and villages tightly clustered in a defensive arrangement that left a largely uninhabited band of terrain between them. Almost certainly these rival city-states were also independent of contemporary polities in the alluvium.

The onset of the Uruk tradition in the Susiana plain was marked by a substantial jump in the number of settlements and the total occupied area trebled in the earliest Uruk phase --- an exponential growth in population which sharply reversed demographic trends of the preceding half a millenium in the area. The end of the Uruk tradition was equally disjunctive. Chogha Mish was abandoned and the size of Susa diminished significantly. If the Uruk presence in the Susiana represented not a process of colonization but rather a functionally specialized intrusion then we would expect to find archaeologically identifiable traces of an alternative but contemporary tradition in the area. Such a divergent tradition has not been recognized.

The Mesopotamian intrusion did not cause the collapse of the indigenous prehistoric cultures of the area. Rather it merely took advantage of an internal process of disintegration that was at the time well advanced. The various surveys of the Susiana Plain indicate that ever since the end of the Middle Susiana Period (late 5th millennium BC) regional population and settlement had been in decline. By the very end of the Late Susiana Period no single site appears preeminent in the Susiana and in terms of economic and political potential the plain was largely undeveloped. Uruk settlers were thus drawn into a fertile and productive area that was only slightly settled and could surely mount only minimal resistance ...